Compounds capable of forming free radicals by photolysis upon exposure to light (free radical forming agent) are well known in the fields of graphic arts and photosensitive recording materials. Such compounds are widely used as a photoradical polymerization initiator in a photopolymerizable composition, as a photoactivator in a free radical photographic composition, and as a photoinitiator in reactions in which an acid produced by exposure to light catalyze. Various photosensitive materials useful in recording systems such as printing, reproduction, duplication and so on, are prepared using the free radical generating agents.
Bis(trihalomethyl)-s-triazine compounds are proposed as halogen free radical generating agents which are sensitive to light of wavelengths ranging from near ultraviolet to visible region. Such compounds are described in detail, e.g., in U.S. Pat. Nos. 3,954,475, 3,987,037 and 4,189,323, and JP-A-62-58241 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Although those compounds are sensitive to light having a wavelength of from near ultraviolet to visible region, they have a problem that their decomposition products upon exposure tend to generate a pale yellow stain, and further, in such a system that a photoradical polymerization initiator or decomposition products thereof remain in a recording layer, a yellow stain due to photolysis generates upon storage under daylight. These phenomena are severe problems in the field such as a color proof, in which a minute change of color tone makes a defect.
As a result of our intensive studies on the above described problems, the inventors have found compounds having excellent properties.